diff --git a/build.sh b/build.sh
index 4579d0d..2e6ef56 100755
--- a/build.sh
+++ b/build.sh
@@ -15,7 +15,7 @@ for version in "${versions[@]}"; do
 	(
 		set -x
 		docker build -t "$base$version-builder" -f "$version/Dockerfile.builder" "$version"
-		docker run --rm "$base$version-builder" tar cC rootfs . | xz -z9 > "$version/busybox.tar.xz"
+		docker run --rm "$base$version-builder" tar cC rootfs . | xz -T0 -z9 > "$version/busybox.tar.xz"
 		docker build -t "$base$version-test" "$version"
 		docker run --rm "$base$version-test" sh -xec 'true'